Shielding flaw detection and measurement in quadrature amplitude modulated cable telecommunications environment

ABSTRACT

Signal egress from a shielding flaw in a cable telecommunication system is detected, even where signals carried by the cable telecommunication system are quadrature amplitude modulated signals that statistically resemble broadband noise by generating a marker signal comprising a double side band, suppressed carrier signal in the fringes of contiguous frequency bands and at a power level which cannot cause perceptible interference with signals in those contiguous frequency bands. The separation of the sidebands comprising the marker signal can unambiguously identify the marker signal and can distinguish between different cable telecommunication systems installed in the same geographic area. The marker signal can be additionally coded by varying the frequency and/or amplitude of the modulating signal used to create the marker signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of Ser. No. 13/080,715filed Apr. 6, 2011, and which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to maintenance of cabletelecommunication systems and, more particularly, to detection of cableshielding flaws and measurement of signal egress in such systems inwhich the communicated signal is quadrature amplitude modulated (QAM).

BACKGROUND OF THE INVENTION

Cable telecommunications systems have been known for a number of yearsand are currently gaining in popularity and coverage for thedistribution of television programming, telephone service and networkingof computers such as providing Internet access since they can carry manysignals over a wide bandwidth with little, if any interference ordistortion, particularly as data transmission rates have increased toaccommodate high definition television, increased volume of digitalcommunication and the like. By the same token, since thesecommunications are intended to be confined within the cable system, theincreased bandwidth required for such communications need not beallocated from the available bandwidth for other communications such asradio, navigation, GPS, emergency communications and the like which mustbe transmitted as electromagnetic waves through the environment.However, flaws in cable shielding in cable telecommunication systems canallow signal egress which can potentially interfere with broadcastcommunications and potentially cause hazards. Reciprocally, flaws incable shielding can permit signal ingress into the cable from theenvironment and degrade or interfere with the signal being carried bythe cable telecommunication system. Therefore, such flaws must bequickly discovered and remedied as they occur due to weather, mechanicaldamage or the like.

Detection of cable shielding flaws is generally achieved throughdetection of the signal carried by the cable transmission system thathas leaked into the environment, essentially by being broadcast from theshielding flaw. Detection of a signal that has leaked or egressed from acable flaw is generally performed in two stages: first, by a receiver ina mobile vehicle driven in the general vicinity of installed cables thatassociates a received signal with a location of the mobile vehicle usinga global positioning system (GPS) receiver which thus reports a generallocation of a shielding flaw and, second, by a hand-held instrument thatcan allow repair personnel to follow increasing signal strength to theexact location of the shielding flaw so that repairs and/or maintenancecan be carried out.

Of course, such detections must be carried out in an environment inwhich noise as well as broadcast signals will also be present in thesame frequency bands. Accordingly, a problem with all such systems is toidentify a received signal as one originating in the cabletelecommunications system and numerous techniques have been developed toeffectively verify or authenticate a detected signal. An additionalissue that follows from this problem is that a signal which is unique tothe cable telecommunication system and distinguishable from broadbandnoise (e.g. a marker signal) necessarily consumes a finite amount ofbandwidth and/or has the potential for interfering with the signalcarried by the cable telecommunication system.

An exemplary system seeking to provide a solution to these relatedissues is disclosed in U.S. Pat. No. 4,072,899, issued Feb. 7, 1978, toRichard L. Shimp, which is hereby fully incorporated by reference. Inthe system disclosed therein, a variable frequency (e.g. “warbled”)audio tone is added as a marker signal to the signal carried by thetelecommunication system which can be easily detected by a narrow bandportable receiver such that the audio tone can be perceived and followedby maintenance personnel while being easily filtered from or havinglittle effect on the other signals carried by the cabletelecommunication system.

However, at the present time, the need to carry ever greater amounts ofinformation (e.g. for high definition television (HDTV) and the like)has resulted in the choice of complex modulation schemes such asquadrature amplitude modulation (QAM) to multiplex signals which are,themselves, more complex and have increased data content. (in general, aplurality of QAM multiplexers (often referred to as QAM generators areused, each carrying a small number of channels of information, and theiroutputs are combined by allocating contiguous spectral bands to each QAMmultiplexer. The output of a QAM multiplexer or a plurality thereof isoften statistically indistinguishable from ambient noise in theenvironment in which detection must be performed.

Accordingly, currently known techniques of signal egress detection haverequired the allocation of significant bandwidth (e.g. the equivalent ofa band corresponding to a QAM multiplexer or at least the bandwidthcorresponding to a television program channel) in order to provide asufficiently complex signal for detection and identification withoutcausing interference with other information carried by the cabletelecommunication system. Allocation of such bandwidth has also beenessential to measurement of the strength of signal egress allowingrepairs to be prioritized and to assure compliance with regulationsgoverning the operation of cable telecommunication systems.

Such an allocation of bandwidth thus reduces the otherwise availablebandwidth of the cable telecommunication system and is essentially alarge fixed cost of operating the system. Even with the allocation ofeconomically significant bandwidth to the shielding flaw detectionfunction, detection is not robust and, where two or more cabletelecommunication systems may be present in the same geographic area,identification of the system having the shielding flaw can often not beperformed unless the marker signal is particularly complex; requiringmore than minimal bandwidth allocation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a cableshielding flaw detection and measurement system which uses a markersignal that cannot interfere with signals carried by a cabletelecommunication system and which does not require allocation ofbandwidth to the shielding flaw detection function.

It is another object of the present invention to provide robustdetection of shielding flaws in combination with a robust andunambiguous identification of the cable telecommunication system fromwhich the egress signal emanates.

In order to accomplish these and other objects of the invention, amethod for detection and measurement of the severity of shielding flawsin a cable telecommunication system is provided comprising generating amarker carrier signal at a frequency at or between the boundaries of twoconsecutively located frequency bands allocated, respectively, to twosources of data to be distributed by the cable telecommunication system,modulating the marker carrier signal to produce two sideband signals asa marker signal, monitoring power of the frequency bands allocated tothe two sources of data and regulating power of the marker signal inaccordance therewith, combining the marker signal with signals in thetwo consecutively located frequency bands to form a combined signal,transmitting the combined signal through the cable telecommunicationsystem, receiving a signal through an antenna, and detecting the markersignal in the signal received through the antenna.

In accordance with another aspect of the invention, a marker signalsource is provided comprising a source of a carrier frequency signal, asource of a modulation signal, a modulator for modulating the carrierfrequency signal with the modulation signal to provide the markersignal, means for monitoring power in a signal band of a signal withwhich the marker signal is to be combined, a comparator for comparingmarker signal power with the power in a signal band of a signal withwhich the marker signal is to be combined, and means for controllingpower of the marker signal.

In accordance with a further object of the inventions, a marker signalreceiver is provided comprising a filter or tuner tuned to a centerfrequency of a marker signal, a spectrum analyzer for determiningspectral content of an output of the filter or tuner, and a comparatorfor comparing a spectrum of an output of the spectrum analyzer with aspectral content of the marker signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1A is an overall high-level block diagram of the Shielding flaw anddetection system in accordance with the invention,

FIG. 1B is a diagram of a portion of the spectrum in which the inventionis employed that is useful in understanding the operation of theinvention,

FIG. 2 is a high-level block diagram of the marker signal source inaccordance with the invention, and

FIG. 3 is a high-level block diagram of the marker signal receiver inaccordance with the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1A, thereis shown a high-level block diagram of the overall cabletelecommunication system 100 in accordance with the present invention.It should be appreciated that the block diagram of FIG. 1A can also beunderstood as a flow chart depicting the methodology of the invention ora data flow diagram. It should also be appreciated that the depiction ofthe telecommunication system 100 shown in FIG. 1A reflects currenttechnology in the constitution of the telecommunication system as wellas integration of the invention into that environment and thus noportion of the drawings is admitted to be prior art in regard to thepresent invention. However, it should be understood that the inventioncan be practiced with legacy (e.g. analog) cable telecommunicationsystems as well as all-digital and other systems which may be developedor foreseen. The application to systems including some or all of thedata input from QAM generators/multiplexers (so denominated since theygenerate a quadrature amplitude modulated signal and provide quadratureamplitude modulation for signals which are generally multiplexed,although no multiplexing of signals is performed in the QAM itself) issimply a particularly challenging environment for shielding flawdetection and measurement in which the invention provides particularlymeritorious effects.

The system 100, as illustrated in FIG. 1A comprises a plurality ofsources 110 of input information. The types of information inputs arenot particularly important to the practice of the invention inaccordance with its basic principles but, at the current time, theplurality of sources may be constituted by a plurality of QAMmultiplexers/generators each receiving digital inputs from a pluralityof sources. The number of sources that can be accommodated largelydepends on the data rate of the input from each source which is oftendigital but analog signal sources can also be accommodated. The outputsof the QAM multiplexers are multi-level signals that are, essentially, ahighly complex analog waveform and thus often statistically resemblesnoise. Thus, the QAM multiplexers may be regarded as not onlymultiplexed signal sources but digital to analog converters, as well. Itshould be appreciated, however, in regard to the operation of theinvention, the QAM multiplexers need not provide any multiplexingfunction and should also be regarded as merely exemplary of data sourcesproviding that data as a modulated signal (not limited to QAM) in asignal over a predetermined frequency band within the spectrum of thecable telecommunication system. The nominal spectra of two adjacent QAMbands are illustrated at A of FIG. 1B.

The signal combiner 120 may be any commercially available unit equippedwith a test point, thus making available an output composite of thecombined signaling for providing a feedback signal 140 to markergenerator 200. The outputs of each of the QAM multiplexers are modulatedto be allocated to a particular frequency band (e.g. of 6 MHz bandwidthin the United States but may be different in other countries) and areindependently connected by plural connections 115 to signal combiner120. These frequency bands will mostly be functionally contiguous (e.g.little information is contained in the “toes” of the bands where theymay, in theory, overlap, as shown at 105 of FIG. 1B) except wherecertain other frequency bands falling within the overall signal spectrumthat can be carried by the cable telecommunication system are allocatedto particular purposes such as emergency warning communications. Theoutput of the signal combiner 120 is then fed to a laser 150 whichproduces a broad band optical signal that is transmitted over fiberoptic link 160 to fiber node 170 where the signal is converted to anelectrical signal for distribution over shielded cables 180 tosubscribers. It is shielding flaws 190 in cables 180 that the inventionis directed to detecting using marker detector 300.

In accordance with the invention, at the head end 130 of the cabletelecommunication system 100, a marker signal source 200 is providedwhich is controllable in regard to the nature of the marker signal thatis to be generated and which also receives feedback 140 from signalcombiner 120 to control the amplitude of the marker signal so thatavoidance of any perceptible interference with other signals can beguaranteed, as will be described in greater detail below. The output ofthe marker signal generator is also supplied as an input to combiner 120which thus outputs a combined signal comprising the data in all of thefrequency bands allocated to the data sources 110 such as the exemplaryQAM multiplexers discussed above.

Referring now to FIG. 2, the architecture of the marker generator willbe discussed in detail. A tunable oscillator 210 provides a carriersignal for the marker (e.g. a marker carrier signal) and is set, inaccordance with the invention, of a frequency, f (FIG. 1B), which is atthe common boundary of two consecutively located and contiguousfrequency bands, A, corresponding to two respective QAM multiplexedsignals. The oscillator output signal or the frequency controlinformation therefor is connected to a QAM band width band pass filter(e.g. 12 MHZ in the United States, as noted above) 215 such that thecenter of the pass band of the filter is the same frequency as thefrequency to which oscillator 210 has been set and at which frequency itis operating. It should be noted that it is not necessary to use asingle QAM bandwidth filter as the same effect can be realized with twohalf-QAM bandwidth (e.g. 6 MHZ) filter devices, each tuned to a centerfrequency (f₁, f₂ in FIG. 1B) of the oscillator 210 frequency, plus orminus one-half of the specified QAM bandwidth allocation. Thus, thesingle or combined band pass filter will pass the signal of twoconsecutively positioned frequency bands corresponding to two QAMmultiplexers in the output of combiner 120 which is fed back to thefilter, as illustrated at 140 in both FIGS. 1 and 2. While the frequencybands chosen are not important to the practice of the invention inaccordance with its basic principles, it is considered generallypreferable to choose a pair of frequency bands which are at relativelylower frequencies in the spectrum of the combiner output. The output ofband pass filter 215 is then fed to a power monitor 220 to evaluate thepower of each of the two adjacent/consecutively located frequency bandsof two respective QAM multiplexers. The power of the frequency bandcontaining the lower power is then communicated to comparator 280 aswill be discussed in greater detail below.

The output of oscillator 210 is also input to a modulator 240 which alsoreceives a modulation signal from a modulation source 230. The output ofmodulation source 230 need be nothing more than a sine wave of knownfrequency but may advantageously be made more complex by varying thefrequency, adding digital coding (e.g. to contain a data payload or thelike, singly or in combination. In some cases, an increased degree ofcomplexity of the modulation signal may assist in unambiguous detectionand identification of the marker amid significantly higher power levelsof QAM signals which, as alluded to above, statistically resemblebroadband noise. Additional coding can provide part or all of suchadditional complexity and provide a conduit for other information todetector 300 for calibration, scaling or any and all other purposeswhich may be deemed desirable for control or operation of the invention,some of which will be discussed in greater detail below while otherswill become apparent to those skilled in the art through experience inusing the invention.

The output 245 of modulator 240 is preferably a double sideband,suppressed carrier signal. That is, the modulation performed bymodulator 240 produces two side band frequencies separated from thecarrier frequency of oscillator 210 by the frequency of the modulationsignal from modulation source 230 indicated at D of FIG. 1B. The carrierfrequency is of no further use and should preferably be suppressed toavoid being a potential source of interference. Thus, the remainingsidebands which will constitute the marker signal are placed within theconsecutively located frequency bands, A, allocated to the two QAMmultiplexers as discussed above where they could be a source ofinterference unless maintained at relatively low power; the reducedpower level being indicated at C of FIG. 1B. It should also beappreciated that, by virtue of the above arrangement, the sidebands alsoare preferably placed in the fringes (indicated at B of FIG. 1B) of thefrequency bands of the two consecutively located frequency bandsallocated to the two QAM multiplexers where they are less likely tocause perceptible interference and may generally be somewhat morereadily detectable. Sideband separation, D, and other unique signaltraits resulting from the modulation source 230 acting on the modulator240 provides the ability to unambiguously identify the emitted signal.This ability to unambiguously identify the marker signal is particularlyimportant and useful to distinguish one cable telecommunication systemfrom another where two or more such systems may have portions thereofinstalled within the same geographical area.

The power of the marker signal is controlled by variable gain amplifier250, the output of which is provided to a directional coupler 260;through or low loss output of which forms the output of the markersignal source 200. The directional or high loss output is fed to a powermonitor 270; an output of which also supplies an input to comparator 280to be compared to the power of the less powerful of two consecutivelylocated frequency bands allocated to two QAM multiplexers as discussedabove. Responsive to these inputs, comparator 280 provides an output tocontrol the gain applied to the marker signal 245 by variable gainamplifier 250 to keep the power of the marker signal lower than thelower powered of the two QAM multiplexers outputs by an amount which hasbeen, for example, empirically determined to avoid any perceptibleinterference with the QAM signal (−30 dB is generally sufficient withcurrent signaling practices and technology).

While not important to the practice of the invention in accordance withits basic principles, it is preferred to also provide logic or circuitryin comparator 280 to detect when control of marker signal power may belost or cannot be kept at a non-interfering power level or possibly whenmarker signal power is of a level that would severely compromise theability to detect a shielding flaw as might occur if the power of thesignal from one of the pair of QAM multiplexers dropped to a very lowlevel. This signal is provided to a latch 290 to control an alarm suchas an indicator lamp, possibly in combination with a concurrentsuspension of generation or injection of marker signals into thecombiner 120, until reset upon correction of the condition that causedthe loss of marker power control or potential interference. As apractical matter for the operator of a cable telecommunication system,it is very important to avoid interference with the signal distributedto subscribers both as a matter of customer satisfaction with thequality of delivered signals but also to avoid generation of servicework orders in response to service complaints or incorrect prioritizingof maintenance work orders based on excessively high marker signal powercausing shielding flaw severity to be erroneously indicated. It isprecisely this strong and multi-faceted requirement for scrupulousavoidance of interference which has required the dedication of valuablebandwidth to shielding flaw detection in (otherwise) QAM environments inpreviously known systems; the avoidance of a requirement for which is amajor meritorious effect of the present invention including provision ofa variable power marker signal that can be kept at a non-interferingpower level. By the same token, the invention assures that the maximummarker signal power level that does not cause perceptible interferenceis provided to optimize and optimally facilitate the detection ofshielding flaws with relatively simple and small mobile/portableinstruments.

Referring now to FIG. 3, a receiver/detector in accordance with thepresent invention will now be discussed. As alluded to above, twodifferent types of instruments are preferably used in different phasesof shielding flaw detection: a receiver in a mobile vehicle that candetect and report a possible egress signal from shielding flaw (and anindication of its location and relative severity) and a portableinstrument to further localize and locate the shielding flaw forconducting a repair and confirming efficacy of the repair made. Bothtypes of instruments have the same architecture as depicted in FIG. 3but will differ in the ultimate output structure 390 a and/or 390 b. Bythe same token, a single instrument could be made to serve bothfunctions but such an embodiment is not preferred since a larger numberof mobile instruments will ensure that far more of the area of cabletelecommunication system 100 is examined for shielding flaws when alarge population of service vehicles are provided with the mobileinstrument and the communications facilities of the mobile instrumentare of significant size and weight and would encumber optimal use of theportable instrument and are unnecessary in the locating of shieldingflaws.

Background noise, including broadcast signals possibly including anegress signal from a cable telecommunication system, are received atantenna 31 and are bandpass filtered and amplified using low noiseamplifier 320. The resulting signal is fed to a tuner 330 tuned to thecenter frequency of the marker source (e.g. the carrier frequency atwhich oscillator 110 is operating). Thus all frequencies are removedexcept those within a small bandwidth surrounding the frequency of thecarrier signal (which has preferably been suppressed as alluded toabove). Thus, the only frequencies which remain are the side band markersignals and the small amount of power in the fringes of the twoconsecutively located QAM signals. These remaining signals are convertedfrom analog form to digital form at A/D converter 340 and the frequencyspectrum is calculated at 350, using a fast Fourier transform (FFT) orother technique such as a spectrum analyzer to allow extraction of thesideband signals. The magnitude of the frequency spectrum is thensupplied to a comparator 360 where they are compared with one or morespectrum templates corresponding to the spectrum (e.g. frequency andseparation, D) of (unique) modulation signal(s) that have been providedin response to modulation source 230, as discussed above. Suchcomparison with templates can also reveal any coding (e.g. sidebandseparation or amplitude) or other signal complexity applied to themarker signal. It should be noted that the matching of templates (e.g.by cross-correlation) provides for discrimination between differentcable telecommunication systems which are in close proximity to eachother. Increased complexity of the marker signal also may provideincreased robustness and facility of marker signal and correspondingshielding flaw detection. Coding of the marker signal (which alsoprovides increased marker signal complexity) might be used to remotelycontrol any aspect of the operation of receiver 300 such as switchingfrom one carrier signal frequency to another or switching betweendifferent marker signal modulation schemes or modes to assuresynchronization of operation of the transmitter 200 and receiver 300 ofthe invention which can provide further increased levels of robustnessin confirming and identifying egress signals. When the degree offavorable comparison between template 390 and frequency spectrum 350exceeds a predetermined threshold, then a measurement of the powerpresent in the frequency spectrum 350 unique to the sidebands of themarker is performed. The peak, over a short time, of the sideband-uniquepower measurement is determined and communicated to the operator,typically using method 390 a or 390 b. In the case of the mobileinstrument using method/instrument 390 b, the location of the peak isalso recorded and communicated. The output of threshold and measurementlogic 380 is either fed to a wireless communication arrangement 390 b ora portable computer for later downloading at a central facility (for amobile type of instrument) or to an on-board display (for a portableinstrument) such that the user can follow increasing signal strength tothe location of a shielding flaw. The information communicated to theoperator may then be used to indicate the severity and location of thedetected leak. One important possible and preferred use of coding of themodulation is to provide an indication of transmitted marker signalpower to the receiver for proper setting of a threshold for detectionand scaling of egress signal power measurement by threshold andmeasurement logic 380 in view of the variable marker signal power levelrequired to guarantee freedom from perceptible interference with thesignal distributed by the cable telecommunication system.

In view of the foregoing, it is seen that the invention provides robustshielding flaw detection in a QAM environment that does not require adedicated spectrum band or frequency in order to avoid interference withsignals distributed by a cable telecommunication system by virtue of themarker signal power being kept below a level which can cause perceptibleinterference with a QAM signal in bands adjacent to the marker signalcarrier frequency. The invention also allows maximum marker signal powerto be utilized relative to QAM signal power while scrupulously avoidingpossible interference with the QAM signal, also by virtue of monitoringthe power of the QAM signals in the frequency bands adjacent to themarker carrier signal frequency and adjusting the marker signal poweraccordingly. The invention also allows unambiguous marker signalidentification by virtue of template matching, possibly augmented bycoding to assure synchronization of the operating mode and marker signalcharacteristics between the transmitter and receiver of the system inaccordance with the invention. The robustness of marker signal detectionand identification amidst high levels of noise and noise-like signalscan be enhanced by virtue of the invention allowing for an arbitrarydegree of increased marker signal complexity. Further, the system of theinvention can be used, in accordance with its basic principles incombination with legacy analog signals or foreseeable all-digitalsystems and other modulation techniques or any combination thereof.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A marker signal sourcecomprising a source of at least two unique signals, a controlarrangement to control said at least two unique signals in frequency andamplitude to avoid interfering with coexisting payload signals beingdistributed in at least one frequency band of a broadband communicationsystem to form marker signals, said frequency of at least one of saidmarker signals being within or adjacent to said at least one frequencyband and at a frequency which is allocated to a portion of saidcoexisting payload signal, and a signal combiner to combine said markersignals with said coexisting payload signals during periods when saidcoexisting payload signals other than control signals are beingdistributed by said broadband communication system.
 2. The marker signalsource as recited in claim 1, further including a comparator thatprovides a signal if power of said marker signal exceeds a predeterminedlevel below said power in said at least one frequency band.
 3. Themarker signal source as recited in claim 2, wherein said comparatorprovides control of one of said source, said control arrangement andsaid signal combiner to terminate generation of said at least two uniquesignals or transmission of said marker signals when power of an adjacentpayload signal band causes control of power of said two unique signalsto be lost or power of said two unique signals cannot be kept at anon-interfering power level.
 4. The marker signal source as recited inclaim 1, wherein said coexisting payload signals are quadratureamplitude modulated signals.
 5. The marker signal source as recited inclaim 1, further including a source of modulation signals.
 6. The markersignal source as recited in claim 5, wherein said source of modulationsignals provides a variable frequency signal.
 7. The marker signalsource as recited in claim 5, wherein said source of modulation signalsprovides a coded signal.
 8. The marker signal source as recited in claim5, wherein said source of modulation signals includes coding to provideadditional data in said two unique signals.
 9. The marker signal sourceas recited in claim 8, wherein said additional data is a control signal.10. The marker signal source as recited in claim 9, wherein said controlsignal corresponds to frequency of said two unique signals.
 11. Themarker signal source as recited in claim 1, wherein said source of twounique signals comprises a source of a carrier frequency signal, saidcarrier frequency signal being of a frequency at boundaries of orbetween two adjacent frequency bands being distributed by said broadbandcommunication system, a source of a modulation signal, a modulator formodulating said carrier frequency signal with said modulation signal toprovide at least two sideband signals as said at least two uniquesignals.
 12. The marker signal source as recited in claim 11, whereinsaid at least two unique signals are a double side band, suppressedcarrier signal.
 13. The marker signal source as recited in claim 1,wherein said two unique signals are located in a fringe of at least oneof two consecutive QAM bands.
 14. The marker signal source as recited inclaim 1, further including logic to control one of said source, saidcontrol arrangement and said signal combiner to terminate generation ofsaid at least two unique signals or transmission of said marker signalswhen power of an adjacent payload signal band causes control of power ofsaid two unique signals to be lost or power cannot be kept at anon-interfering power level.
 15. A marker signal receiver comprising afilter or tuner tuned to a center frequency between at least two uniquesignals having frequencies differing from each other and from saidcenter frequency, said at least two unique signals constituting a markersignal, said two unique signals being controlled in amplitude to anon-interfering level below an amplitude of payload signals coexistingin time and frequency with said two unique signals in a broadbandcommunication system, said filter or tuner having a passbandapproximately equal to a frequency separation of said two uniquesignals, a spectrum analyzer for determining spectral content of anoutput of said filter or tuner, and a comparator for comparing aspectrum of an output of said spectrum analyzer with a spectral contentof said marker signal, wherein said marker signal receiver is responsiveto coding of information into said marker signal to control an aspect ofoperation of said marker signal receiver, and wherein said aspect ofoperation of said marker signal receiver controlled in response to saidcoding of said marker signal includes synchronization with modulation ofa marker signal from a marker signal source.
 16. A marker signalreceiver comprising a filter or tuner tuned to a center frequencybetween at least two unique signals having frequencies differing fromeach other and from said center frequency, said at least two uniquesignals constituting a marker signal, said two unique signals beingcontrolled in amplitude to a non-interfering level below an amplitude ofpayload signals coexisting in time and frequency with said two uniquesignals in a broadband communication system, said filter or tuner havinga passband approximately equal to a frequency separation of said twounique signals, a spectrum analyzer for determining spectral content ofan output of said filter or tuner, and a comparator for comparing aspectrum of an output of said spectrum analyzer with a spectral contentof said marker signal, wherein said marker signal receiver is responsiveto coding of information into said marker signal to control an aspect ofoperation of said marker signal receiver, and wherein said aspect ofoperation of said marker signal receiver controlled in response to saidcoding of said marker signal includes change of said center frequency.17. The marker signal receiver as recited in claim 16, wherein saidspectrum analyzer is constituted by an analog-to-digital converter and afast Fourier transform processor.
 18. The marker signal receiver asrecited in claim 16, wherein said comparator compares said spectrum witha spectral template corresponding to said spectral content of saidmarker signal.